1. Field of the Invention
The present invention relates to the storage of ammonia in a solid form using metal ammine complexes and delivery there from. The invention also relates to the use of ammonia stored in a solid form as the reducing agent in selective catalytic reduction (SCR) of NOx in exhaust gases from combustion processes, to methods for producing such complexes and to an ammonia delivery device comprising such complexes.
2. Description of the Related Art
Current environmental regulations necessitate the use of catalysts in the exhaust gas from automotive vehicles, boilers and furnaces for control of NOx emission leaving the system. Particularly, vehicles equipped with diesel engines or other lean burn engines offer the benefit of improved fuel economy, but suffer from the drawback of increased formation of NOx being noxious and which must be eliminated from the exhaust gas. However, catalytic reduction of NOx using conventional so-called three-way exhaust catalysts for automobiles is impossible because of the high content of oxygen in the exhaust gas. Instead, selective catalytic reduction (SCR) has proven useful for achieving the required low levels of NOx in the exhaust gas both in stationary and mobile units. In such systems NOx is continuously removed from the exhaust gas by injection of a reducing agent into the exhaust gas prior to entering an SCR catalyst capable of achieving a high conversion of NOx. So far, ammonia has proven to be the most efficient reducing agent, which is usually introduced into the exhaust gas by controlled injection of gaseous ammonia, aqueous ammonia or aqueous urea. In all cases, the amount of reducing agent being dosed has to be very precisely controlled. Injection of a too large amount of reducing agent will cause emission of ammonia with the exhaust gas whereas injection of a too small amount of reducing agent causes a less than optimum conversion of NOx.
In many mobile units powered by combustion engines, the preferred technical solution is to use an aqueous solution of urea as the reducing agent since in this way potential hazards or safety issues relating to the transport and handling of liquid ammonia in high pressure containers are eliminated. However, there are several disadvantages related to the use of aqueous urea as the reducing agent. First of all, the use of urea solutions requires the carrying of a relatively large volume in order to provide sufficient amounts of ammonia to allow a vehicle to drive e.g. about 20,000 kilometres without having to substitute or refill the source of ammonia. In typical systems, an aqueous solution comprising about 30 wt % of urea is preferred meaning that about 70 wt % of the content of a container holding the urea solution is used only to transport water. During the decomposition, one molecule of urea forms two molecules of NH3 and one molecule of CO2 and thus, ammonia only constitutes roughly 50 wt % of the weight of the urea molecule and hence, the concentration of ammonia of the reducing agent is very low. Similar concentrations of ammonia can be achieved using aqueous solutions of ammonia as reducing agents. Furthermore, when using solutions, specially designed spray nozzles combined with a precision liquid pump are required to ensure that a) the aqueous urea is delivered to the exhaust system at a desired (and dynamically changing) flow rate and b) the aqueous urea is efficiently dispersed in the gas phase before entering the catalyst. Finally, the aqueous solutions might freeze in cold weather conditions (below minus 11° C.), or the urea solution may simply form precipitates, which will block the dosing system, e.g. the nozzle. Altogether, these difficulties may limit the possibilities of using SCR technology in abatement of pollution from NOx, particularly in connection with mobile units.
Transporting ammonia as a pressurized liquid is hazardous as the container may burst or a valve or tube might break in an accident giving a discharge of poisonous/lethal gaseous ammonia. In the case of the use of a solid storage medium containing absorbed/adsorbed ammonia, the safety issues are much less critical since a small amount of heat is required to release the ammonia from the storage medium.
International Patent Publication No. WO 99/01205 discloses a method and a device for selective catalytic NOx reduction in waste gases containing oxygen, using ammonia and a reduction catalyst. According to the method, gaseous ammonia is provided by heating a solid storage medium comprising one or more salts, especially a chloride and/or sulphate of one or more cations selected from alkaline earth metals, and/or one or more transition metals, preferably Mn, Fe, Co, Ni, Cu, and/or Zn, said storage medium being introduced into a container. In the preferred embodiments of WO 99/01205 the cation is Ca2+ or Sr2+. The inventive method and device are stated to be particularly suitable for use in automobiles.
However, the use of the ammonia storage media known from WO 99/01205 suffers from various draw-backs hampering a wide-spread use in the automotive industry. The vapour pressure of ammonia above a solid salt phase is e.g. about 1 bar at room temperature and atmospheric pressure for calcium octa ammine chloride and strontium octa ammine cchloride complexes rendering the use somewhat complicated due to the high pressures that must be taken into account in view of the toxicity of ammonia. Having such a high partial pressure of ammonia, the handling and trans-portation of the saturated storage material is still difficult and also dangerous. Still further, the use of granulated storage materials—stated as preferred embodiments in WO 99/01205—requires measures for preventing the storage material from leaving the container during use in moving vehicles. Furthermore, a granulated material will have a considerable void between the granules which will drastically reduce the bulk density of the storage material by a factor 1.5-2.
It has now been found that these drawbacks may be overcome using a magnesium salt complex according to the present invention providing a compact, light-weight, cheap and more safe storage for ammonia having a very low vapour pressure of ammonia below 0.1 bar at room temperature to be used in the automotive industry.